In the fields of aircraft and aerospace industries, the development of new materials which are very light and yet strong has been requested recently.
Therefore, high strength and high modulus inorganic fibers have been developed using ceramic, graphite or boron and these have been utilized. However, on the other hand, they have not been widely applied because they were very expensive and difficult to handle.
But, it was found that aromatic polyamide fibers had high strength, high durability to friction and high modulus, and thus the development of them has progressed actively.
Also, they have been used as reinforcing materials for plastics in various fields of industry such as the aircraft industry, etc., especially, resin composites reinforced with glass fibers have been used as building materials.
But, there were many problems with the foregoing fibers and films. In an effort to solve the prior problems, West Germany patent laid open No. 1,810,426 to Kwoleck discloses a method for increasing the tensile strength and modulus of the fiber.
The method comprises wet-spinning using aromatic polyamide dope having an optically anisotropic property, and heating the fiber while applying tension.
Thus, aromatic polyamide dope was prepared according to the above method, and spun to obtain fibers and films therefrom. In particular, pulp-like short fibers by the above method were expected to be very widely employed as insulating materials, adiabators, and friction resistant materials in place of asbestos.
As mentioned above, aromatic polyamide fibers, especially pulp-like short fibers having many advantages have been studied and many improvements in their properties made, consequently the following methods have been invented.
That is, according to U.S. Pat. Nos. 3,869,429 and 3,869,430, high molecular weight polymer is obtained by dissolving inorganic salts and aromatic diamines in amide solvents, polymerizing with aromatic diacid chloride at a low temperature to make the polymer of the acid crumb form, washing to remove the polymerization solvents and inorganic salts, and continuously drying. Liquid crystalline dope is obtained by redissolving the above polymer in 20% by weight sulfuric acid, chlorosulfuric acid or flourosulfuric acid.
This dope is then spun through a spinning hole to water, neutralized, washed, dried and treated by mechanical processes to prepare the object aromatic polyamide fibers and film. In the application of the prior art, there are many disadvantages in that the process is complicated, and therefore, costly.
The manufacturing apparatus is easily corroded by sulfuric acid, and also the process is somewhat dangerous. Because the polymer decomposes in dope, the property of fibers may be reduced.
Moreover, not only is it difficult to handle calcium sulfate (CaSO.sub.4), a by-product, but also the color of fibers changes with time due to residual sulfuric acid.
The compression strength of the fiber of this method is much weaker than the theoretical value, due to a defect layer in the fiber itself, and its chemical resistance is also poor because a crystal defect layer is formed in the fiber.
In Japan patent publication So 59-47694, aromatic polyamides of low molecular weight according to interfacial polymerization are dissolved in an amide solvent, and the resulting solution is mixed with a precipitator and stirred to produce pulp-like particles. In this method, this polymer is precipitated and stirred in water or alcohol, the inherent viscosity of the fiber is about 2 to 3.5 at most, therefore the property is decreased and high quality aromatic polyamide short fibers can not be easily prepared.
Also, recent art U.S. Pat. No. 4,511,623 discloses a novel method for preparing highly oriented and high strength aromatic polyamide short fibers having an inherent viscosity of 5 to 7. These fibers are prepared by adding tertiary amines like pyridine prior to the polymerization reaction and applying a high mechanical shear rate (160 cm.sup.-1 ) at the moment gelation is achieved, hence, rapidly increasing the rate of polymerization and producing a high degree of orientation and a high degree of polymerization.
But in that case, because the amide solvent and excess tertiary amines are added simultaneously, the viscosity is suddenly increased while polymerization occurs by means of adding terephtaloyl chloride. Therefore, polymerization is rapidly terminated within 10 seconds, and then control of the process is very difficult. It is impossible to continuously produce fibers.
U.S. Pat. Nos. 3,672,143 and 3,817,941 disclose art analogous to the present invention. Those are the methods where the polymerization media as a dope (This dope is In situ polymer dope and is different from Isolated polymer dope.) is spun or ejected by extrusion into a precipitator, and then the object fibers are obtained by said simple process. On the other hand, they have some disadvantages in that it is difficult to increase the molecular weight due to the use of a chain terminator during polymerization and water as a precipitator. Moreover, it is impossible to obtain high quality fibers with an improved molecular weight.
The prior method for preparing aromatic polyamide fibers comprises polymerizing, washing, preparing 20% by weight dope of high molecular weight PPTA (poly-(p-phenylene terephthalamide)) and concentrated sulfuric acid, and spinning into water through an air gap to produce fibers For instance, this method is based on the principle that the orientation of the molecular chain of the high molecular weight polymer is achieved by drawing during spinning using the foregoing concentrated (20%) solution. And though the molecular chain of the aromatic polyamide is rigid and the polymer forms liquid crystals, the complete orientation of the molecular chain can not be achieved due to the high viscosity of more than 1,000 poise at 80.degree. C.
The resulting aromatic polyamide fibers have microscopic structure of skin (exterior) and core (interior) portions as shown in FIGS. 6(a) and (b). The core portion has crystal defect layers every 250 nm perpendicular to the fiber axis and a radially arranged pleated sheet structure, while the skin portion has a compact and well-oriented structure. (Ref. Journal of Polymer Science; Polymer Physics Edition, Vol. 21, 1955-1969 (1983)). The fiber strength is considerably lower in comparison with the theoretical strength because of the crystal defect layers perpendicular to the fiber axis of the core portion.
A report in the Journal of Polymer Science; Polymer Physics Edition 21, 1757 (1983) discloses a case of spinning after preparing sulfuric acid dope. But in that case, the chain ends of the polymer are ionized to --NH.sub.3.sup.+ HSO.sub.4.sup.- and form clusters which comprise the defect layers to the fiber axis. When a force is applied, weak crystal defect layer is formed, hence, chemical strength and durability decrease. In said paper, it is stated that the strength can be increased by preventing cluster formation, however, it is impossible to attain this by prior art methods.
In the case of polyethylene PE), the strength of existing fibers is less than 5 g/d, however, if the molecular chains are extended by a specific spinning known as gel spinning, the strength is increased to at least 20 g/d. And, though the chemical structure of two compounds is the same, their properties are remarkably different from each other because of their crystalline structures.
Therefore, the present inventors have concentrated on the foregoing point and have sought to improve the property of aromatic polyamides for a long time. Hence the present invention.
An object of the present invention is to provide aromatic polyamide fiber and film having high strength and a high degree of orientation by means of separating the "liquid crystalline pre-polymer dope" (LCPD) formation process from the "molecular orientation polymerization" process, permitting the final polymerization and the molecular chain orientation to occur simultaneously in order to overcome the prior disadvantages, including the economic disadvantages of a multistep process, the difficulty of process control due to a sudden increase in the degree of polymerization, the low degree of polymerization, etc.
Another object is to provide a process for the preparation of aromatic polyamide fibers and film in which the manufacturing cost is inexpensive and process control is easy, and the fiber can be produced successively and obtained directly at the same time as polyermization.
Another object is to provide aromatic polyamide fibers and film with an inherent viscosity of higher than 3.0, with random distributions of the molecular chain's end, where a crystal defect layer is not formed perpendicular but parallel to the fiber axis, and also the color is not changed by aging.